Crank shaft PK50XL

Unfortunately the crank shaft of my PK50XL elestart (V5X3T, 1987) got damaged. The pre compression plate has also been damaged. Now I want to install a reed valve intake manifold with full cycle racing crank shaft to get the engine back to life. Problem is that I can only find a 19mm manifold (Polini), the original carburettor is a SHB16.10 for which a 16mm manifold is required. 

Question: Is it possible to install a 19/19 carburettor on the 50cc (racing) cylinder or do I also need to increase the cylinder capacity?

Hope somebody could advise what would be best to do before I start buying the wrong stuff.